This invention relates to the transmission of sound in the walls of vessels, such as those used for containment of liquids in nuclear reactors, for the detection of flaws within the material of the walls and hidden below the surface of a wall. More particularly, the invention relates to the positioning of plural sets of transmitters and receivers in an array for observation of such flaws from different directions.
At many industrial sites, particularly at nuclear reactors, large vessels are constructed for the containment of liquids such as water. The vessels may be formed as tanks for holding the liquid at one location, or in the form of conduits for transporting the liquid from one location to another. These vessels are frequently constructed of a steel wall, sections of which may be welded togethr during the construction process. For example, the base metal of the wall may be a carbon steel which is sufficiently strong for holding the liquid under pressure. In addition, the wall may be clad with a protective skin such as a layer of 1/4 inch thick stainless steel. The stainless steel protects the vessel from corrosive substances that may be present within the liquids. In particular, it is noted that great care is required in assembly of the vessels to minimize all chances of failure including failures due to stresses of high pressure.
A problem arises in that a flaw may occur in the material of the wall, particularly at the site of a weld. Such flaw may be hidden from view by the outer cladding of the stainless steel skin. One attempt at the detection and location of such flaws has employed the use of a sonic transmitter and receiver which are mounted in a side-by-side arrangement for direction of sonic energy towards the flaw, and for reception of sonic energy reflected back from the flaw. With the foregoing arrangement, the flaw lies on a plane located between the transmitting and receiving transducers. As a result, the rays of energy tend to focus primarily upon the flaw and reduce the possibility of detection of sonic echoes at a location other than at the flaw. The two transducers must be scanned together in different directions about the surface of the vessel wall to ensure detection of a flaw. Such a procedure may be excessively time consuming and also may introduce a risk that the scanning misses a possible flaw.